Tularemia is a zoonotic disease that causes geographically confined
and seasonal outbreaks in many locations in the Northern Hemisphere
(1-3). The highly infectious causative bacterial agent, Francisella
tularensis, comprises 4 subspecies, but nearly all cases of tularemia
are caused by subspecies tularensis (type A), the most virulent type,
which is found in North America, or subspecies holarctica (type B),
which is the most widespread species in Europe (4). F. tularensis can
infect humans through bites of arthropods (e.g., mosquitoes, ticks,
tabanid flies); inhalation of infectious aerosols; handling of infected
animals; or ingestion of contaminated water (2,3).

Sweden, Finland, and Turkey have reported the highest incidences of
tularemia worldwide (5). In Sweden and Finland, the most common form of
the disease is ulceroglandular tularemia, which is characterized by a
skin ulcer at the site of infection and adjacent swollen regional lymph
nodes (6-8). A marked seasonality of tularemia has been reported in
Sweden; most cases occur during late summer and early autumn (8-12). An
exception was an outbreak affecting >2,700 persons during late fall
and winter in 19661967 (13). In 2000, large numbers of cases were
recorded outside the historically tularemia-endemic northern regions of
Sweden, which could indicate a changing geographic pattern of disease
(6).

F. tularensis subspecies holarctica naturally infects several
mammalian wildlife species, in particular, mice, rabbits, hares,
beavers, voles, lemmings, and muskrats (14). In Europe, the ticks
Dermacentor reticularis and Ixodes ricinus are vectors for the bacterium
(15-17), although previous research has suggested that mosquito bites
are the most frequent route of transmission to humans in Sweden
(6,9,18). Furthermore, a relationship between exposure to F. tularensis
and the presence of lakes and rivers has long been suspected and is
repeatedly described in the literature on tularemia (4,19-23). However,
the ecologic cycles and environmental reservoirs of tularemia remain
largely unknown. Since the 1950s, observed disease patterns have
suggested that tularemia foci in nature coincide with a suitable
ecosystem at a particular place (21,24,25). According to this theory,
disease vectors, hosts, and the pathogen are tied to a particular
landscape--that is, an ecologic region--as the environmental determinant
that controls disease distribution.

To determine the ecologic factors that contribute to the
transmission of F. tularensis and the spread of tularemia in Sweden, we
examined trends in the epidemiology of tularemia among humans during
1984-2012. We analyzed descriptive epidemiologic data, including the
geographic distribution of cases during the study period, and
investigated if changes in distribution occurred and if disease was
associated with particular ecologic regions and inland water.

Methods

Sources of Epidemiologic, Geographic, and Demographic Data

Since 1968, suspected and confirmed tularemia cases have been
mandatory reportable diseases as required by the Swedish Communicable
Disease and Prevention Act. Data on tularemia cases occurring from
September 1984 through December 2012 were collected from the national
system for communicable disease surveillance database, maintained by the
Public Health Agency of Sweden. Data on patient sex, age, place of
residence, suspected location of disease exposure, date of exposure,
date of onset of illness, date of diagnosis, and date of notification
were retrieved from this database. This study was approved by the
Regional Ethical Review Board in Umea, Sweden (2014-204-31M).

Data on population, land, and water areas were retrieved from
Statistics Sweden (26). The proportion of land area covered by inland
water (defined as lakes and rivers >6 meters wide) was determined by
municipality. The areas of the 4 largest lakes of Sweden (Vanern,
Vattern, Malaren, and Hjalmaren) and sea water areas were calculated
separately by municipality. Incidences of tularemia, nationwide and by
municipality, were calculated on the basis of the number of infections
per 100,000 persons and year by using population census data for
1984-2012. The altitude (meters above sea level) at which each tularemia
case-patient was exposed to F. tularensis and the centroids of the 9,875
postal code areas in Sweden were retrieved by using the 30 arc-second
digital elevation model of Europe (27) and the extraction toolset of the
Spatial Analyst toolbox in ArcGIS version 10.0 (ESRI, Gavle, Sweden).
Geographic data were visualized by using ArcMap software in ArcGIS and R
software version 2.9.1 (http://www.r-project.org).

Geographic Coding and Spatial and Temporal Data

For each tularemia case recorded, geographic coordinates
(longitude/latitude) were determined for the location of disease
exposure and the disease onset date. (For details, see online Technical
Appendix, http://wwwnc.cdc.gov/EID/article/21/1/14-0916-Techapp1.pdf.)
These data underwent quality coding to enable subsequent analysis with a
high level of spatial and temporal certainty.

Ecologic Regions and Definition of Northern and Southern Sweden

For our study, we used previously defined ecologic regions (e.g.,
areas defined by the distribution of flora, fauna, geomorphology,
climate, and soils) (28). The southernmost part of Sweden is nemoral
forest (broadleaf forest); north of this region is the boreo-nemoral
forest region (mixed deciduous and coniferous forest). Most of the rest
of the country is boreal forest (coniferous forests) and is divided into
3 subregions: southern, middle, and northern boreal forest. Alpine
tundra is located in northwestern Sweden and is composed of the birch
forest, the middle to low alpine forest (grass and shrub heaths, fens),
and the high alpine forest (boulder fields). We defined southern Sweden
as the region located south of the southern border of the boreal forest
region; the area above this boundary was defined as northern Sweden
(29).

Local Outbreaks and Outbreak Length

A local outbreak was defined as >4 cases of tularemia during a
30-day period in a municipality. The criterion for a new outbreak was a
lag phase of [greater than or equal to] 4 months after the end of the
preceding 30-day outbreak period. The mean duration of outbreaks was
compared between the northern and southern parts of Sweden by measuring
the interquartile ranges of the outbreaks (i.e., the periods during
which 50% of cases occurred).

Statistical Analysis

Categorical data were analyzed by using the [chi square] test for
goodness of fit. Differences between incidence proportions were analyzed
by using the 2-tailed 2-proportion z-test with a 95% CI, and
nonparametric bootstrapping was used to construct a 95% CI for the
relative increase in risk. The Wilcoxon rank-sum test was used to
compare differences between groups, and the Spearman rank correlation
was used to study the dependencies between variables. The spatial
distribution of tularemia cases was compared with a regularly
distributed set of points determined by the underlying population of
each municipality. A municipality was part of an ecologic region if its
geographic centroid was within the borders of the ecologic region. All
statistical analyses were conducted in R version 2.9.1.

Results

Epidemiologic Characteristics of Tularemia in Sweden

During 1984-2012, a total of 4,830 cases of tularemia were notified
to the Public Heath Agency of Sweden; of these, 4,792 patients were
infected in Sweden. The annual mean incidence of tularemia on the basis
of these 4,792 patients was 1.86 cases per 100,000 persons (range
0.00-5.62; Figure 1). A total of 2,791 (58.2%) case-patients were men;
mean age was 47.6 [+ or -] 19.5 years (men, 47.8 [+ or -] 19.5; women,
47.2 [+ or -] 19.5; range 1-95 years). After applying quality criteria
for disease onset date and location of disease exposure, 3,524 of the
4,792 cases were included for the subsequent analyses (the quality of
descriptive metadata connected with cases is summarized in online
Technical Appendix Table 1). All further results, including incidence
estimates, were determined on the basis of these 3,524 cases for which
high-quality data on disease onset date and location of infection were
available.

During the study period, tularemia incidence was distributed widely
among age groups, with the highest incidence among those 55-69 years of
age (Figure 2). The mean annual incidence by age group showed a distinct
bimodal pattern for both sexes; peaks in incidence for age groups 10-14
and 55-59 years were 0.93 and 2.75 cases per 100,000 persons,
respectively. The global relative risk of contracting tularemia was 1.39
times higher for men/boys than for women/girls; the corresponding
difference in incidence between sexes was 0.44 cases/100,000
persons/year (95% CI 0.35-0.53; p<0.001). By age group, the incidence
of tularemia for the study period was significantly higher for men than
for women in all age groups [greater than or equal to] 55 years of age
(Figure 2). The male:female relative risk for infection was 0.89 for the
0- to 4-year age group but ranged from 1.15 to 4.28 for all other age
groups.

Spatial and Temporal Distribution of Tularemia Cases

Tularemia reports were highly seasonal. The cumulative number of
cases per week for 1984-2012 showed a symmetric pattern with a peak at
week 32; approximately two thirds of cases occurred during weeks 29-35
(mid-July to late August; Figure 3). The seasonal outbreak peaks were
similarly distributed in the southern region (weeks 30 to 35) and in the
northern region (weeks 30 to 34; Wilcoxon rank-sum tests, p>0.05).
The mean lengths of outbreaks were also similar between regions
(Wilcoxon rank sum test, p>0.05).

Tularemia was reported in 189 of 290 municipalities during the
study period; some geographic clustering of cases was evident (Figure
4). Most cases were reported from the northern region, where
[approximately equal to] 20% of the Swedish population lives, and
incidence was significantly higher in the northern region (4.52
cases/100,000 persons/year) than in the southern region (0.56
cases/100,000 persons/year). The 95% CI for estimating the difference in
incidence between the northern and southern regions was 3.77-4.14
cases/100,000 persons/year (p<0.001). Denser case aggregates were
found in northeast areas of Sweden and in a belt around the southern
border of the boreal forest region, which includes the municipalities
Ljusdal, Malung, Ockelbo, and Orebro (Figure 4).

The nationwide number of local outbreaks per year varied from 0 to
12 (mean 2.86 [+ or -] 3.80) and was largely correlated with the
nationwide annual incidence, demonstrating that many local outbreaks
occurred simultaneously during outbreak years (Figure 1). The number of
outbreaks per municipality during 1984-2012 ranged from 0 to 9 (online
Technical Appendix Table 2). The highest annual tularemia incidence per
municipality was recorded in Ockelbo in 2000 (921 cases/100,000
persons), followed by Malung in 2003 (588/100,000 persons) and Ljusdal
in 2008 (429 cases/100,000 persons) and 1998 (402 cases/100,000 persons)
(Figure 4).

Analysis showed a significant long-term change in the annual mean
incidence of tularemia from the first to the second half of the study
period. Incidence was 0.26 cases/100,000 person/year during 1984-1998
but 2.47 cases/100,000 persons/year during 1999-2012 (Table). The 95% CI
for estimating the difference in incidence between 1984-1998 and
1999-2012 was 2.16-2.36 cases/100,000 persons/year (p<0.001). An
analysis of incidence by municipality showed that, during the first half
of the study period, tularemia was mainly reported from municipalities
in northern Sweden (Figure 5). However, as the nationwide tularemia
incidence increased in the second half of the study period, the disease
occurred over a larger geographic area, extending into the southern
region. The rate of increase in case reports during the study period was
9.6 times higher in the south than in the north ([chi square] test, 95%
CI 6.37-16.93; p<0.001).

Tularemia and Ecologic Regions, Altitude, and Inland Water

The incidence of tularemia was unevenly distributed among the 6
ecologic regions of Sweden (Figure 6). An even disease distribution
based on 3,524 cases within the country corresponded with a mean
incidence of 1.35 cases/100,000 persons/year. The observed incidences in
the nemoral, boreo-nemoral, and combined boreal and alpine regions were
0.02, 0.80, and 4.61 cases/100,000 persons/ year, respectively. The 95%
CI for measuring the deviation from the mean incidence was -1.42 to
-1.23 cases/100,000 persons/year (p<0.001) for the nemoral region;
-0.62 to -0.47 cases/100,000 persons/year (p<0.001) for the
boreo-nemoral region; and 3.05-3.47 cases/100,000 persons/year
(p<0.001) for the combined boreal and alpine region.

Exposure to F. tularensis occurred at a median altitude of 59.0
meters (range 0-900 meters); the densest case aggregates were
distributed over different altitudes (Figure 7). For municipalities that
reported tularemia cases, the mean incidence was 9.27 cases/100,000
persons/year, and a significant positive correlation was seen between
the mean altitude of disease exposure and disease incidence (Spearman p
0.41, 95% CI 0.28-0.53; p<0.001). The incidence was significantly
higher than expected at altitudes >100 meters and lower than expected
at altitudes <50 meters (p<0.001). The 95% CIs for measuring the
deviation from the mean incidence were 1.08-2.42 cases/100,000
persons/year at altitudes >100 meters and -1.80 to -0.77
cases/100,000 persons/year at altitudes <50 meters.

We found a positive correlation between the mean incidence of
tularemia and the proportion of municipality area covered by inland
water (Spearman p 0.36, 95% CI 0.23-0.47; p<0.001) but a negative
correlation between the mean incidence of tularemia and the proportion
of the municipality areas covered by sea water (Spearman p -0.28, 95% CI
00.40 to -0.15; p<0.05). We found no correlation between the mean
incidence per municipality and the proportion of the municipality area
covered by the 4 largest lakes in Sweden (Spearman p -0.10; p>0.05).

Discussion

We used 29 years of nationwide notifiable disease surveillance data
on tularemia in Sweden to investigate the epidemiologic patterns in
relation to time, disease location, and certain ecologic factors. The
approach enabled us to identify a marked overrepresentation of cases in
the northern part of the country, including annual incidences of 400-900
cases per 100,000 persons in some municipalities, and to identify a
marked increase in the overall number of cases reported during the study
period. We also observed that the rate of disease increase was higher in
the southern part of the country than in the northern part, which
suggest that tularemia is becoming more common in the southern regions.
We also found statistical support for an association of tularemia with
the location of lakes and rivers and with certain ecologic regions of
Sweden, a finding notable because the existence of such associations has
been postulated for decades but did not have robust support.

We determined that those 40-70 years of age were at greatest risk
for tularemia, whereas, for unknown reasons, young adults were at the
lowest risk and children and young teenagers were at intermediate risk.
A similar age distribution was recently reported from Missouri, USA,
which indicates that this type of age distribution is not unique to
Sweden (30). Age-related differences in disease incidence could mirror
differences in behavior; for example, higher-risk outdoor activities
such as farming, hunting, and berry-picking may be more common among
40-70-year-olds.

The difference in incidence that we found between the sexes, with
tularemia being more common in men than in women, is notable but not
unique to this study. Similar findings were reported in several earlier
studies (7,13,31). It is unclear if the sex bias results from a
difference in exposure to disease or if men are simply more susceptible
to tularemia. Our results demonstrate that differences by sex occurred
across age groups for all case-patients [greater than or equal to] 5
years of age, a finding that suggests that, among the possible biologic
mechanisms involved, sex hormone differences are unlikely to explain the
difference in incidence.

Results from our large set of case data corroborated previous
reports on the seasonality of tularemia. The disease risk peaks in late
summer or early autumn with most cases occurring in early August
(6,7,9). In addition, we found that tularemia incidence was almost
10-fold higher in the second half than in the first half of the study
period. We cannot rule out the possibility that improved disease
awareness accounts for this difference, but because tularemia typically
carries distinct symptoms, this is unlikely.

In agreement with previously published data, we observed that the
tularemia incidence was highest in the northern part of Sweden and that
the disease distribution was uneven, with some municipalities reporting
multiple outbreaks (8-10,12). The high risk of contracting tularemia in
some municipalities is noteworthy and indicates that tularemia is a
public health issue in these locations. Twelve municipalities recorded
maximum annual incidences of >100 cases per 100,000 persons and
infection transmission occurred almost exclusively during a few summer
weeks, findings that offer several possibilities for enhanced preventive
measures. Future research efforts can lead to the development of
outbreak prediction tools that can help public health authorities make
timely decisions on campaigns informing the public on steps to take to
prevent infection, such as avoiding exposure to mosquito bites (18).

During the second half of the study period, risk for tularemia
increased 9.6 times more in southern areas than in northern areas, which
indicates that the disease is becoming more common in the southern part
of the country. From the available data, we cannot determine whether
this shift is occurring because F. tularensis is dispersing to new areas
or if more infections occurred in the south during the second period
because ecologic conditions facilitated increased bacterial
multiplication and spread (e.g., through infected arthropod vectors).

A link between tularemia caused by F. tularensis subspecies
holarctica and the location of lakes and rivers was suggested by
extensive field work investigating tularemia >50 years ago (19-22),
but we could find little data to prove this association. We found that
tularemia incidence at the municipality scale was positively correlated
with the proportion of land area covered by inland water (lakes and
rivers) but negatively correlated with the proportion of land area
covered by sea water. The latter finding can be interpreted as a
relatively low incidence of F. tularensis infection in municipalities
with a long coastline, a finding which contrasts with the distinct
association of F. philomiragia, F. novicida, and some other Francisella
spp. with sea water environments (4).

Interpretation of our finding of underrepresentation of F.
tularensis exposure at altitudes <50 meters and overrepresentation at
altitudes >100 meters is difficult. The most dense geographic case
aggregates from 1984-2012 were observed over a range of altitudes, which
indicates that intense disease transmission to humans appears to be only
marginally restricted by altitude. To provide more informative data,
future studies should target areas that experience repeated outbreaks of
tularemia and aim for detailed analyses of local disease exposure
altitudes and proximity to lakes and rivers. Compilation of chemical and
physical characteristics of lake and river waters in areas where disease
incidence is high might clarify what kinds of aquatic ecosystems are
connected with F. tularensis.

We also examined a possible correlation of tularemia with ecologic
regions, sometimes referred to as a landscape epidemiology of tularemia
(21), and found that tularemia cases were overrepresented in the 3
boreal forest regions and the alpine region of Sweden. Combined with the
findings described above, these data support a scenario in which the
disease is closely related to certain (micro-) environments and ecologic
systems (21,24,25).

The strengths of the study include the large sample size and
29-year study period; of the total number of 4,792 cases, 3,524 cases
had high-quality descriptive metadata on date of disease onset and
location of disease exposure. In 1967, Pollitzer provided a complete
overview of the published literature on tularemia in the Soviet Union
with greater total case counts, but the raw data used in these older
studies are difficult to compare with modern infectious disease
surveillance data (32). Limitations of our study include the inherent
weaknesses of infectious disease surveillance data in general; for
example, data on risk factors related to human behavior are lacking, and
disease in humans may be underreported because of few clinical symptoms
(33,34). Some inaccurate or imprecise information on disease onset date
and location of disease exposure may also have slipped through our
filters to ensure strict data quality for case inclusion (e.g., because
of patient recall bias).

In conclusion, our findings should stimulate discussion on future
possibilities to prevent tularemia. Although this disease does not cause
a high number of deaths, the illness can be incapacitating for days,
weeks, and sometimes even months. Future studies should focus on the
causes of an increased risk for disease in men and in older persons of
both sexes. Our findings of a significantly increased risk for
contracting tularemia in certain ecologic regions and the positive
correlation between disease and inland water may prove useful in future
prevention strategies. We believe that knowledge of ecologic region and
proximity to water can be used to define areas within which tularemia
exposure is more likely. In addition, dynamics introduced by climate
change, such as increasing temperature and changing precipitation
patterns, can be incorporated in risk assessments (18,35). Finally,
further study is needed to identify the reservoirs of F. tularensis in
nature and the role of vector abundance. Human activities such as the
restoration of wetlands and changes of land use may affect tularemia
incidence, but data to influence appropriate risk assessments are
lacking.

DOI: http://dx.doi.org/10.3201/eid2101.140916

Acknowledgments

We thank all physicians in Sweden and the epidemiologists at the
Public Health Agency of Sweden who provided and curated the
epidemiological information in the national communicable disease
surveillance system and the personnel at Statistics Sweden and at the
Swedish Meteorological and Hydrological Institute for providing open
data for research. We also thank Frauke Ecke for providing the map on
ecological regions.

This work was supported by grants from the Swedish Research Council
for Environment, Agricultural Sciences and Spatial Planning (Formas no.
2012-1070) and Vasterbotten County Council (Dnr. VLL-378261).